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RESOLVING THE FERMI PARADOX
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RESOLVING THE FERMI PARADOX
Rudy E. Kokich, Alexandra Kokich, Andrea Hudson
7 July 2018
In 1975 astronomer Michael Hart proposed the Fermi Paradox, implying a contradiction between the lack of direct evidence for extraterrestrial intelligence (ETI) and the presumed probability that they exist in substantial numbers. Recently, Oxford researchers Sandberg, Drexler, and Ord applied the Monte Carlo simulation to the Drake equation, and concluded that there is up to 99.6% probability that we are alone in the Milky Way galaxy, and up to 85% probability that no other intelligent life exists in the entire observable universe.
We submit that, from physical, chemical, biological, and statistical perspectives, these conclusions require Earth to occupy a singular environment in the universe which is estimated to contain one hundred billion trillion habitable exoplanets. Although there is no direct evidence for ETI, we propose that sufficient indirect evidence exists to reasonably hypothesize a profusion of simple extraterrestrial life and considerable numbers of intelligent life. We resolve the Fermi paradox by showing that the premise of the paradox represents a fallacy in informal logic, and that the paradox is a false dichotomy easily explained by insufficient investigation. By calculating the mean interstellar distance between presumed numbers of ETI in the galaxy, we show that, except by pure coincidence, the nearest ETI would lie hundreds or thousands of light years (LY) away. If highly advanced civilizations are also subject to the universal laws of nature, then traversing such distances, or even attempting radio communication, presents unrewarding if not insurmountable technological, social, and economic challenges. The impracticality of such enterprises is reinforced by the consideration that any possible proceeds would not arrive for scores of generations.
After a flurry of UFO sightings, during lunch with colleagues at Los Alamos National Laboratory in 1950, physicist Enrico Fermi estimated the probability of intelligent alien life in the galaxy, and raised the question, "Where are they?" If there are tens of billions of Sun-like stars with Earth-like planets, and some of them evolved intelligent life, why is there no record in human history of extraterrestrial visitors or their artifacts?
According to a Scientific American article by Robert H. Grey , Fermi's question was declared a "paradox" in 1975 by astronomer Michael Hart. He proposed that, if technological aliens existed, "they would inevitably colonize the Milky Way" either as individuals or with self-replicating robotic probes. The label Fermi Paradox, and the conclusion "... they are not here, therefore they do not exist," became accepted as popular wisdom, and were actually cited by Sen. William Proxmire in 1981 and Sen. Richard Bryan in 1993 to deny government funding for NASA's SETI program.
Written accounts of the 1950 Los Alamos lunch by physicists Emil Konopinski, Edward Teller, and Herbert York indicate that Fermi did not present his question as a paradox, and did not challenge possible existence of alien civilizations. According to York, Fermi "went on to conclude that the reason that we hadn’t been visited might be that interstellar flight is impossible, or, if it is possible, always judged to be not worth the effort, or technological civilization doesn’t last long enough for it to happen.”
Since 1992, nearly four thousand exoplanets have been detected, with 632 star systems having multiple planets. As our detection methods are mostly restricted to systems with planetary orbit planes in our line of sight, it is certain that we presently can not document a great majority of exoplanetary systems. Still, every year of research reveals the Solar System to be far from unique, and that most, if not virtually all, high metallicity Population I stars have planets, some of which are suitable for life. Nevertheless, the Fermi paradox, better renamed Hart Paradox, remains entrenched in popular and scientific thinking, asserting doubt in the existence of alien civilizations due to the absence of any positive evidence such as radio signals, extraterrestrial visitors, or robotic probes.
The Drake Equation and the SETI Model:
In 1961, several days prior to the first scientific meeting on the search for extraterrestrial intelligence (SETI), radio astronomer Frank Drake wrote an equation including seven variables which need to be considered when estimating the number, N, of civilizations in our galaxy capable of radio communication at the current time. 
N = R x Fp x Nc x Fl x Fi x Fc x L
R = rate of star formation in our galaxy.
Fp = fraction of those stars with planetary systems.
Nc = number of habitable zone planets per star system.
Fl = fraction of suitable planets on which life actually appears.
Fi = fraction of life bearing planets on which intelligent life emerges.
Fc = fraction of civilizations which develop suitable radio technology.
L = the length of time such civilizations release detectable signals.
During the original SETI meeting, attended among others by eminent astronomers Carl Sagan and Otto Struve, the seven parameters in the equation were largely based on conjecture. Educated guesses gave the minimum value for N as 20, and the maximum value of 50,000,000. The maximum value was skewed by the maximum estimate for L of 100 million years - assuming a geological scale for the lifetime of a technological civilization.
Since then, research has revealed much more precise estimates for the first three parameters in the equation. According to NASA and the European Space Agency, the rate of star formation in our galaxy ® is 1.5 - 3 stars per year, the fraction of stars with planetary systems (Fp) approaches 1, and the fraction of stars with planets in habitable zones (Nc) is roughly 0.2, or one in five. 
However, at the current stage of scientific development, there is no observational basis to assign values to the next three parameters. It is not known how often life develops on suitable planets, how often such life becomes "intelligent", or how often intelligent life develops technology. As for the lifetime of a technological civilization, it has been convincingly argued that it might last indefinitely if it overcame all threats to its survival, or that it might obliterate itself within several centuries or even decades.
Consequently, the Drake equation, written by Drake's own declaration as an agenda for a meeting, to stimulate scientific dialogue, can not be used to draw reliable conclusions regarding the number of technological civilizations in a galaxy. Depending on the assumed values for the unknown terms, the result may be from far less than one to tens of millions. Such a wide ranging approximation does not seem to be a useful scientific result as much as an estimate of the observer's pessimism or optimism.
The Oxford Model:
In a paper titled Dissolving Fermi Paradox, submitted to the Proceedings of the Royal Society of London, and published online at arXiv.org , three researchers at Oxford University's Future of Humanity Institute claim to have calculated significant probability that we are the only technological civilization in our galaxy, and even in the entire visible universe. The authors, Sandberg et al, treated each parameter in the Drake equation not as a single best guess point but as a series of uniformly selected values from a range of historically estimated uncertainty. Monte Carlo simulation  then yielded 53% - 99.6% probability that we are the only technological civilization in our galaxy, and 39% - 85% probability that we are alone in the observable universe. They declared the Fermi paradox dissolved since our failure to detect aliens is no longer in conflict with the expectation that they exist. In their own words, “we find a substantial ex ante probability of there being no other intelligent life in our observable universe, and thus that there should be little surprise when we fail to detect any signs of it."
Although the authors approached the Drake equation with a more sophisticated statistical model, their results may be no closer to the unknown "reality" than the SETI Drake model. Their estimates of current scientific uncertainties, and therefore ranges assigned to the Drake parameters (in their Table 1), are highly debatable. Presenting the same results to show the probability that an extraterrestrial intelligence (ETI) does exist yields the lowest value for the the galaxy of
100% - 99.6% = 0.4%.
In a universe of 200 billion galaxies, the probability that ETI does exist would be:
1 - ( 0.004^2E11 ) = 1, or 100%, not 15%.
The Frank-Sullivan Model:
For yet another approach, consider the article A New Empirical Constraint on the Prevalence of Technological Species in the Universe  published by Frank and Sullivan in the journal Astrobiology. In order to remove unknown parameters from the Drake equation, Frank and Sullivan redefined the ETI question. "Rather than asking how many civilizations may exist now, we ask, are we the only technological species that has ever arisen?" Their equation becomes:
A = N x F , where:
A is the number of technological species which emerged over the lifetime of the universe,
N is the number of habitable planets in the universe, and
F is the fraction of habitable planets on which technological species emerge.
Although simple, the equation can not be solved since parameters A and F remain highly speculative. The authors then redefined the question to find the probability against humanity being the only technological society which has ever developed in the universe. Parameter A now becomes 1, and the equation becomes:
F = 1 / N
The European Space Agency, estimates there are roughly 10^22 - 10^24 stars in the universe.  According to the latest data from the Kepler space mission, approximately one in five stars has a planet in the habitable zone. Assuming the middle value of 5x10^23 for the number of stars in the universe, the number of habitable planets, N, is:
N = ( 5x10^23 ) / 5 = 1x10^23
and the probability, F, that humanity is the only technological civilization which has ever existed in the universe is:
F = 1 / ( 1x10^23 ) = 1x10^-23
or about one in one hundred billion trillion !
On the scale of our galaxy, assuming the number of stars to be 3x10^11 :
N = (3x10^11) / 5 = 6x10^10
F = 1 / ( 6x10^10 )
or about one in sixty billion !
However, these numbers are not as optimistic as they at first appear. Since the universe is 13.8 billion years old, if every single habitable planet in the galaxy eventually developed a technological civilization, that number would be 60 / 13.8 = 4.3, only about four per year. The estimated number of ET civilizations existent at the present time then strictly depends on the estimated longevity of a technological civilization - parameter L in the Drake equation. For supposed longevity of 1,000 years, the maximum number of contemporary galactic civilizations would be 4,300.
Validity of Statistical Models:
Various statistical models predict a wide range of outcomes: from no ETI in the entire universe to millions in our galaxy alone. Since all models depend on authors' assumptions and estimates rather than objective data, the fundamental question remains whether any statistical method can return valid information regarding an entity about which no factual knowledge exists. The only certitude, known from Earth's experience, is that a technological civilization is possible. The results obtained by the Oxford researchers may be only a reflection of the degree of their pessimism. In comparison, ETI probability evaluation made at the first SETI meeting, although equally uncertain, seems to have been drawn by a highly optimistic group. Whether we accept a universe in which we are absolutely alone, or one which is teeming with intelligent life depends at this time entirely on our subjective intuition of what is "reasonable". And, what is reasonable depends to a major degree on the three unknown parameters in the Drake equation: Fl, Fi, and Fc.
Estimates of Three Drake Parameters:
Knowing only Earth's example, we have no information to define Fl, the fraction of suitable planets on which life actually appears. On Earth, the earliest evidence of life in the form of fossilized microorganisms was found in Canadian sedimentary rocks, interpreted as submarine hydrothermal vent precipitates, and dated between 3.77 and 4.28 billion years old.  In another study, UCLA geochemist Mark Harrison found carbon isotope evidence of life in 4.1 billion year old zircon crystals from western Australia. These studies extend by up to 300 million years the generally accepted age of Earth's oceans (3.8 billion years) which are thought to be necessary for the emergence of life.
Numerous planetary missions have revealed that geological features like volcanoes, mountains, lake beds, sea basins, river deltas, sedimentary rocks, and erosion due to wind and heavy rainfall are present on other planets. We are now developing instruments capable of detecting in the atmospheres of exoplanets indirect evidence of alien life like methane, oxygen, and complex organic molecules. In fact, such molecules have only recently been reported in the plumes emerging from Saturn's moon Enceladus, suggesting the possibility of a deep-ocean thermal vent biosphere.
As years go by, it is ever more apparent that there is nothing unique about our Sun, the Solar System, or the Earth itself. There seems to be nothing exclusive about our geology, chemistry, or physics. Since the laws of nature observed on Earth are equally valid elsewhere in the universe, it seems irrational to cast heavy doubt on the possibility that organic chemistry on suitable exoplanets will evolve into primitive life. The evidence that life appeared only several hundred million years after Earth's formation 4.54 billion years ago might imply that life is plentiful throughout the universe, and dispose us to assign higher values to parameter Fl - perhaps close to 1.
Estimating parameter Fi, the fraction of life-bearing planets on which intelligent life emerges, is more difficult. If the passage of time between two related events reflects the probability of the second event, then on Earth it took about 3.5 billion years for early life to develop primordial intelligence in the form of fish. This process took nearly a quarter of the age of the universe, and involved a number of complex stages:
1) the emergence, by prokaryotic endosymbiosis, of eukaryotic cells with a nucleus and membrane-bound organelles, about 2 billion years ago ;
2) the emergence of **** reproduction about 1.1 billion years ago;
3) the appearance of multicellular animals in the fossil record about 600 million years ago ; and
4) the appearance of an elementary brain in bilaterally symmetrical animals, roughly 550 million years ago. 
Another 350 million years passed before the mammals, the first class of animals we would consider conventionally "intelligent", developed the cerebrum, or neocortex, which is associated with higher cognitive functions. 
In the Earth's example, the long time frame involved in the emergence of intelligence implies a low value for parameter Fi but, in the absence of many additional samples, does not suggest any specific quantity.
Before evaluating a "reasonable" estimate for parameter Fc, the fraction of planets on which intelligent species develop technological civilizations, we need to consider issues related to human self-perception.
In part due to human nature, and in part to incomplete observational evidence, throughout history mankind has always made the initial assumption that it is unique, and occupies a special place in creation. Since the beginning of time this tendency heavily influenced our political history, socioeconomic development, and science - especially cosmology and biology.
Initially we believed in the Ptolemaic geocentric system, in which Earth occupied the center of the universe. Science gradually demoted us to a small planet around a random star, in an ordinary galaxy, arbitrarily located among several hundred billion in the visible universe, which itself may be only an insignificant fraction of an infinite multiverse.
In the field of biology, mankind also made the initial assumption that it is unique. Until recently scientists were trained to strictly guard against anthropomorphism. Man was presumed to act by reason and independent thought, and animals only by instinct. However, latest research by behavioral and neuro-scientists reveals that higher animals also perceive physical sensations and a spectrum of emotions just like we do. They observe, think, learn, communicate, use tools, form societies, and modify their behavior in a logical manner consistent with self-interest. Human distinction appears to be a matter of degree rather than the kind of ability. If we abandon the perspective that we are biologically unique, we can observe that on Earth only about two hundred million years - a blink of the cosmological eye - separate the first animals with a complex brain from the first technological civilization.
This implies a rather high value for Drake parameter Fc, perhaps also approaching 1.
The most time consuming, and presumably least probable, stages in the evolution of life on Earth were the emergence of eukaryotic cells and the emergence of multicellular animals. Avoiding the temptation of excessive self-regard, and using Earth as a model for other habitable planets, probabilities favor a galaxy teeming with primitive life, but with low incidence of even elementary intelligence. Yet, where primitive brains do develop, complex brains evolve in 350 milion years, and a technological civilization follows in about 200 million.
An interesting proposition was put forth at the first SETI meeting: that with some Drake parameters high, like R, and some low, like Fi, the product of the first six parameters is very approximately equal to 1, and the number of contemporary technological civilizations in our galaxy, N, is numerically equivalent to L, the number of years such civilizations persist. Considering the Earth model, this proposition is not entirely unreasonable - within an order of magnitude. Since Homo sapiens emerged about 300,000 years ago , with this approach the number of alien species of approximately human intelligence might be tens or even hundreds of thousands in the galaxy.
However, that does not answer Drake's question. He specifically asks how many technological civilizations are detectable. Of the ones which have the ability, how many are actually beaming powerful radio signals precisely in our direction, over very long periods of time, attempting a contact? That number is likely to be far, far lower.
Clearly, estimated numbers of alien civilizations will vary widely based on the initial conditions imposed by very specific questions. What is intelligence? What is technology? What is detectable? Do aliens have to be biological, or can they be composed of semiconductors, in which case their longevity as individuals and as a species might be much greater?
Challenging the Fermi Paradox:
In an article An Explanation for the Absence of Extraterrestrials on Earth published in the Quarterly Journal of the Royal Astronomical Society in 1975, astronomer Michael Hart wrote: "We observe that no intelligent beings from outer space are now present on Earth. It is suggested that this fact can best be explained by the hypothesis that there are no other advanced civilizations in our galaxy."  This concept, labeled Fermi paradox, was predicated on the assumption that technological aliens, if any ever existed, would have inevitably colonized the entire galaxy over millions of years.
The so called Fermi paradox has been challenged on many levels , most of which are reasonable and a few conspiratorial. But one obvious question has to come first. Is Michael Hart's statement logical?
The first part of Hart's contention: "ETI are not observed, therefore ETI are not here" is false because it requires omniscience on the part of the observer. ETI might very well be here and not observed because the observer has limited perceptions. The second part: "ETI are not here, therefore ETI do not exist" is also false because the premise (ETI are not here) may be false, and because ETI may very well exist in many places, but not specifically here. If Hart's contention is false, the paradox does not exist.
As the ancient Romans originally reasoned, absence of evidence does not imply evidence of absence. No valid conclusion can be inferred from the absence of evidence because evidence may be lacking due to insufficient investigation. Indeed, we have not yet made a detailed investigation of our own solar system, much less of the galaxy or the universe.
Logical error in Hart's Fermi paradox implies an invalid approach in using that line of reasoning to explain lack of contact with the aliens, or to estimate the probability of their existence.
Is Earth Unique in the Universe?:
At this stage of our scientific knowledge any discussion of intelligent aliens, or even of alien life, constitutes pure speculation. This does not mean the speculation is mere guesswork. We do have some indirect information on which to base an educated opinion. With relatively high confidence, we know the following:
-Life developed very early in the history of our planet.
-The probability of life increases in proportion to the number of suitable environments.
-Life is highly adaptable to extreme environments and to competition.
-Probability of complex organisms increases with the passage of time.
-Evolutionary pressures favor the emergence of the brain and intelligence.
-Some intelligent species can develop technology.
-We have observed that natural laws appear to be universal.
-We presently know that most, if not all, stars have planetary systems, and that approximately one in five have planets in the habitable zone. There are likely tens of billions of suitable planets in the galaxy, and billions of trillions in the universe.
-Although we are unique as spiritual individuals, there is nothing exclusive in our collective physical existence as a species.
Assuming Earth is not singular, but a random sample of habitable environments in the universe, an educated argument in favor of intelligent aliens vastly outweighs any argument against their existence.
How Far are the Aliens?:
Mean interparticle distance equations can approximate mean distance between star systems potentially populated with extraterrestrial intelligence. In a three dimensional space model, where V is the volume of a cube with a specified side S, and N the number of ETI contained in that volume, mean distance between ETI, D, is given by:
D = ( V / N )^1/3 where V = S^3
However, the galaxy is roughly 100,000 LY in diameter, and on average only 1,000 LY in thickness. The largest three dimensional cube which can be drawn within the galaxy would have a side of 1,000 LY, and a volume V = 10^9 cubic LY. Since the approximate volume of the Milky Way is 8x10^12 cubic LY, the galaxy can be divided into 8,000 such 1,000 LY cubes. On the small scale, where the mean distance between ETI stars is less than about 1,000 LY, sections of the galaxy are most accurately modelled in three dimensions.
On the large scale, the galaxy as a whole has the appearance of a flat pancake, and is best modelled in two dimensions as an area, A, of a circle with a radius R = 50,000 LY. The equation then becomes:
D = ( A / N )^1/2 where A = p x R^2
As the chart below shows, which scale, or volume model, is used depends on the total proposed number of ETI in the galaxy. When the number reaches 8,000 the mean distance between ETI is 1,000 LY, and the volume model can be shifted to three dimensions.
Illustration 1: Mean distance between ETI based on the proposed ETI number in the galaxy.
The results are shown in the semi-log graphical form in illustrations 2 and 3 below.
Illustration 2: Mean distance between ETI using the 2D model of the entire galaxy.
Illustration 3: Mean distance between ETI using the 3D 1,000 LY cube model.
The results are quite sobering. If there were 10 alien civilizations in the galaxy, the mean distance between them would be 28,025 LY. If there were 16,000, the distance would be 794 LY. As the 3D model shows, there would have to be 80 million ETI in the galaxy before we could expect to have a "local neighbor" within about 50 LY. Even by the most optimistic estimates the likelihood is that the nearest ETI lie hundreds, if not thousands, of light years away. This is reason enough to understand why the probability of any type of contact is vanishingly small.
Impediments to Contact:
Physical contact would involve traversing at relativistic velocities unimaginably vast distances through interstellar space filled with high energy plasma, strong cosmic radiation, dust, meteoroid material, and even undetectable black holes, rogue planets and comets. With our current technology and scientific knowledge we can not even hypothesize how - or even why - such voyages might be undertaken by biological or artificial species. In practical terms, millenia upon millenia would pass before any rewarding information could return to the home planet.
It is true that presently, at the very dawn of the space age, our theoretical and technical abilities are expanding at an exponential rate. But, at some point science will encounter the law of diminishing returns, and the learning curve will become sigmoid. We should not expect that anything we can now imagine will eventually become possible through science and technology. It is likely that even the most advanced civilizations are not able to violate the basic laws of nature by travelling faster than light, traversing space through "wormholes", moving backward in time, or altering the gravitational constant. In that case, alien space travellers are as constrained as we are by the laws of physics and biology, and by the economies of time and resources.
At present, the most promising method of detecting ETI is by searching for their radio signals. We have been transmitting broad-band, omnidirectional radio broadcasts for about a hundred years. In theory, our signals have already reached about 500 local stars. However such signals are probably not detectable beyond a fraction of a light year due to rapid dissipation and strong cosmic interference. Historically, SETI scientists have been searching for very narrow-band signals in the microwave region of the electromagnetic spectrum between 1 and 9 GHz. This region has relatively little background cosmic noise, and contains two narrow emission lines, including hydrogen at 1.420 GHz and methanol at 6.667 GHz, which are felt to be universal "radio dial markers". So far, SETI research has been unsuccessful. Even a narrow-band signal is not detectable at great distances unless it is emitted by a very powerful transmitter, and directed in a tight beam precisely in our direction. Then, the signal has to last long enough to be accidentally detected by SETI's narrow, star by star search. 
Even an optimist recognizes very low probability of detecting a powerful beacon aimed specifically in Earth's direction by an ETI which is eager to eternally advertise its presence at substantial economic cost, with indeterminable risk, and with extremely low probability of a reply delayed by many generations.
In the end, the Fermi paradox as defined by Hart, which infers that ETI do not exist because we have no evidence of them, is a fallacy in informal logic because it relies on obviously insufficient investigation to promote its conclusion. Yet, the question, originally asked by Fermi himself, of why there is no record in human history of extraterrestrial visitors or their artifacts, is quite valid, and central to the formulation of hypotheses for further research.
Over the years, numerous answers to Fermi's question have been offered in scientific and popular literature. Some of them are rational, and others outlandish. The most plausable answer, already proposed by Fermi, seems to be the most obvious. Interstellar travel at relativistic speeds over distances of hundreds or thousands of light years is technically not practical, economically unproductive in terms of expense / reward ratios, and socially unintersting since required time frames call for scores of lifetimes of biological and even artificial life forms. Under these constraints, radio contacts are also unlikely because that would involve sustaining complex, expensive projects over thousands of years, with very unpromising prospects.
It may be suggested that scientific progress in more advanced civilizations might eventually solve some of the problems which to us presently seem theoretically insurmountable. That may not be the case. The growth of scientific knowledge can not forever accelerate at an exponential rate. It will eventually encounter the law of diminishing returns, slow down, and gradually approach some upper limit - hopefully in a very distant future. Technological capabilities of very advanced ETI might be far ahead of ours, and appear "miraculous", but are not likely to be infinite. Advanced aliens' knowledge of physics would probably refine but not invalidate our modern physics, just as Einstein's refined but did not invalidate Newton's. We should therefore not expect advanced aliens to possess technology which allows them to violate the fundamental laws of nature as we understand them today. In essence, like us, they will be subject to the same basic constraints of speed, space, time, and fundamental physical constants. This is the simplest explanation for why we have not had any proven alien contacts.
The existence of ETI does not depend on our knowledge or ignorance of them. They either emerged or did not emerge. Statistical models attempting to estimate the number of ETI based on indirect evidence implied in the Drake parameters produce results ranging from millions of ETI in the galaxy to none in the entire universe. Alhough it is impossible to quantify how these models reflect reality, the Oxford model which estimates high probability that we are alone in the visible universe implies a related conclusion: that Earth is somehow unique among the estimated one hundred billion trillion habitable exoplanets. On other suitable planets either life never arises, or it does not evolve into more complex forms, or it never develops intelligence, or intelligence never matures into science and technology. Making such an extraordinary claim carries the burden of extraordinary proof, far beyond numerical outcomes of a statistical model.
In fact, astronomers are commonly making discoveries that, compared to other stars in the galaxy, the Solar System and Earth are not exceptional in the physical or chemical sense. Rather than a singular case, it is almost infinitely more likely that Earth represents a random sample of habitable planets, and that processes observed here are common elsewhere in the universe. It follows that it is most reasonable to propose the hypothesis that on habitable exoplanets simple life exists in profusion, and intelligent life in unknown but "substantial" numbers.
And, what of the average lifespan of a technological civilization? Extinction rates from the fossil record suggest that the average lifespan of an invertebrate species is around 11 million years, while a mammal species lasts about 1 million years.  In addition to the dangers from the natural biological, environmental, geologic, and cosmic catastrophies, the survival of a technological society is threatened by the capacity for self destruction. But while science and technology can be an existential threat, they also allow for self-preservation. Improvements in agriculture, medicine, and socioeconomic condition have allowed the human race to become the dominant species on Earth. Further benefits to survivability may lie in diversification by constructive bioengineering, development of artificially intelligent life forms, and colonization throughout home-star systems. Such enterprises would disperse risk, enable a species, or its various versions, to achieve great longevity, and significantly increase the total number of contemporary ETI.
Finally, the answer to Fermi's question is different for the optimists and the pessimists.
Fermi already provided answers for the optimists. The galaxy may indeed be teeming with intelligent aliens but, in Herbert York's words, Fermi thought that "...interstellar flight is impossible, or, if it is possible, always judged to be not worth the effort, or technological civilization doesn’t last long enough for it to happen.”
For the pessimists who claim we might be the only technological civilization in the galaxy and even in the universe, in strictly scientific terms we must admit ignorance in view of complete absence of direct evidence. But, we also have to qualify that ignorance. Due to the vastness of the universe and the relative modesty of our instruments, our investigations have been astonishingly limited. To paraphrase astrophysicist Neil deGrasse Tyson, we have dipped a glass into the ocean, looked into it, and declared, "There are no whales."
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